Hasn't anyone done this? I've been considering ideas myself. We're going to be building a coop for our chicks, and already thinking about next winter. I did find a "solar heater" you put in a window on www.shopthecoop.com tonight, even before I saw this post, but what I had been talking to my husband about was somehow being able to hook up a heat lamp using solar power? Not wanting it 70 degrees in there or something, but perhaps above freezing to keep the chill out somewhat, and keep the water from freezing. I would love to see a discussion on this!
Solar heat, eh? South facing windows which allow light to shine on a dark wall or floor ought to heat it up. If the wall/floor is masonry, it should last a long time after the sun goes down. Getting the heat should be the easy part. Regulating the temperature seems like the real problem.
Er, you would need a GIANT bank of collectors and batteries to do it that way. Heat lamps draw a huge amount of current. It is not really practical. Notice that even proper, state of the art house-sized solar systems do NOT use solar energy to run electric-powered heat
If you were going to do it, at least use your giant bank of panels and batteries to run something that produces *only* heat rather than wasting some of the output on light (heat lamps are NOT the most efficient way to produce heat!)
But since what you want is *heat*, and since what comes in through the window is in large part *heat*, there is no reason to involve collector panels and batteries at all. All you need to do is get as much heat as possible a) into the coop or part thereof, and b) to stay there for as long as possible during the night. Always accompanied by c) good insulation so's to make best use of what residual heat is available thru the night. The last of those is perhaps the most important.
You can use a heating panel, either store boughten like the one from shopthecoop.com or kludged together yourself for much less, to heat the coop during the day. Or you can just put thermal mass (see below) (preferably dark colored) somewhere that light from the windows will warm it up during the daytime. Either way, the more thermal mass you have in the coop, the more of the daytime heat you will be capturing for the chickens to benefit from at night (and the less severely the coop will overheat during the day).
Things that have large thermal mass include water -- like, inside of black-painted plastic jugs or 55 gal drums -- or rock, or concrete. (Or gravel, but its use in a chickenhouse is probably impractical).
But it would take SERIOUS engineering to do the same good during wintertime nights that a heat lamp would do, in terms of the temperature in the coop. Basically you would have to build a coop-sized version of one of the best available solar heated *houses*. With additional complications of not being able to have bare rock or cement floors in the coop, and it being a smaller building and thus harder to keep warm.
Also, if you are in a Cold Climate you may have to use insulating panels that you manually place over the windows at night... those same windows that let sun in also let heat *out* rather briskly when it's dark out. This can be a big nuisance, esp. if you are trying to maximize the hrs of daylight that layers get and don't want to run a lightbulb.
Really, if you want a passive means of keeping the coop from getting too cold in the winter, far and away your best bang-for-your-buck comes from creating an extremely well insulated coop, even to the point of building it cordwood or strawbale construction style. The chickens THEMSELVES are fairly effective furnaces, and the more of their heat you keep in (while still allowing ventilation) the warmer it will be.